Not a nit, more an observation - tricorders sure have come a long way in the last three years. When Crusher wanted to confirm whether or not Jason was Picard's son in "Bloodlines", she had to do all sorts of complicated tests that took ages. In this episode Dax scans Miranda with a tricorder and can instantly confirm that she is an O'Brien. It's not even a medical tricorder, either.

Plotline

While heading back to Deep Space Nine after a long patrol, the Defiant stops to check out a planet surrounded by a mysterious energy field. They are surprised to be greeted by their own descendants on the surface - a settlement founded by the Defiant crew who were stranded on the planet two centuries earlier after the ship was thrown back in time and crashed while trying to pass through the barrier on the way home. Knowing the fate that awaits them, the Defiant crew are confident they can avoid the accident - but will they really go forward knowing that it will guarantee that eight thousand people will never have existed?